Gas analysis method



1967 J. M. MCKEE 3,347,635

GAS ANALYSIS METHOD Filed May 20, 1955 lo FE G. 2 '5 *AR= Increase infilament r 08 resis1ance(ohms)during first minute of exposure 1 0 6 atconstant current.

I00 200 300 400 500 INVENTOR Concentration of Reactive ComponenHppm. byvol.) BY John McKee fyyfWATTORNEYS United States Patent 3,347,635 GASANALYSIS METHOD John M. McKee, Arrnonk, N.Y., assignor to United NuclearCorporation, White Plains, N.Y., a corporation of Delaware Filed May 20,1963, Ser. No. 281,716 3 Claims. (Cl. 23-232) The present inventionrelates to a method of detecting the presence of and measuring theconcentration of impurities present in a nominally inert gaseousenvironment. More particularly, the invention provides a novel method bywhich the presence and concentration of chemically reactive impuritiesis related to the change in electrical resistance of a filament due toreaction with the impurities in a nominally inert atmosphere.

In recent years there has been a substantial increase in the number andvariety of practical applications of materials which are chemicallyreactive with elements in the normal atmosphere. Of necessity, thesematerials must be worked in controlled environments in order to precludeundesirable chemical reactions. For example, the welding of refractorymaterials is usually performed in an impervious enclosure filled withinert elements such as helium or argon. However, it has been found thatthese inert gases, even though initially pure, often become contaminatedwith active impurities. In order to minimize the costly deleteriouseffects of these foreign substances their presence must "be promptlydetected and their coii'centrations must be readily and accuratelyascertained.

There are known methods and apparatus for making determinations of thiskind. For example, a known method includes the step of positioning ametallic filament in the controlled environment, heating the filamentand then, with suitable instrumentation, either measuring the timerequired for the filament to burn through, or measuring the heatgenerated by the chemical reaction between the filament and theimpurities in the gas. In practice, the application of this techniquehas proved laborious, inconvenient and somewhat inaccurate. In order tosimplify and improve the qualitative and quantitative determination ofreactive components of a nominally inert gas I have invented an accurateand easily performed method which requires only simple and inexpensiveinstrumentation.

According to my invention my new method comprises the steps of placingin a nominally inert gaseous atmosphere to be tested a body of asubstance which is chemically stable at normal atmospheric temperatures.The substance is then heated by suitable means in order to render thesubstance relatively unstable. Further, in accordance with my newmethod, any change in electrical resistance etween separated points onthe body of the substance is determined as it reacts with theimpurities, if any, in the gas being tested. The measured value of thechange in resistance of the substance is then related to that valueassociated with known concentrations of specific impurities in anominally inert gas.

I have found that functional plots of resistance with respects to timeat any given value of heating current have generally the same shape overa substantial range of concentrations of the reactive or impuritycomponent, but the scale is contracted or expanded depending on Whetherthe concentration of the reactive component is large or small. Theresistance varies nonlinearly as a function of time, increasing at agreater rate during the initial stages of the heating cycle thansubsequently. I have further found that, for a resistive substance ofgiven dimensions and for a given substantially constant value of heatingcurrent, the rate of change of resistance with 2 respect to time in theinitial stages of the heating cycle -curately ascertain theconcentration of the combining gas in the inert atmosphere.

A feature of my invention is that it may be performed without complexelectrical apparatus; rather, my new method may be practiced withseveral kinds of known electrical circuits and components. I have foundthat a simple bridge circuit with appropriate rheostats, resistors,relays and timing instruments can be used to practice the method quiteadequately. More complex apparatus may be employed if it is desirable torepeat the test of an atmosphere at frequent intervals and if automaticoperation is also desirable.

In the following specification I give a detailed description of aparticular method according to my invention. In the course of thisdescription reference is made to the accompanying drawing in which:

FIG. 1 is a schematic diagram of electrical circuitry having utility inthe practice of a particular method according to my invention, and

FIG. 2 is a graph of changes in resistance plotted againstconcentrations of water in helium.

Referring now to the drawing, the apparatus selected to illustrate thepractice of my new method comprises an enclosure 10 adapted to contain anominally inert gaseous atmosphere. This enclosure may be a well known,so-called glove box. Within the glove box or other enclosure there aresuitable means for supporting a chemically reactive metal filamentindicated schematically at 11. There are provisions for connecting twoseparated points on the filament to external electrical circuitry as forexample, by leads 12 and 13. A particular means for supporting afilament within the enclosure, including a filament stringing apparatuswhich is suitable for use in the practice of the present invention, isdescribed in my copending application, Serial No. 203,142, filed June14, 1962, now Patent No. 3,148,033. It is sutficient to say here thatthe apparatus described in the copending application is capable ofstringing new filaments quickly and efliciently and Without anynecessity for opening the gas filled enclosure.

As is illustrated schematically Within the broken outline 14, I providea resistance bridge circuit adapted to be energized from an alternatingcurrent source. The reactive metal filament is connected in one leg ofthis bridge circuit as will be described subsequently. The filament isenergized through the bridge circuit and the bridge circuit also servesto detect changes in resistancev of the metal filament when it isenergized and reacting with impurities in the nominally inert gaseousatmosphere within the enclosure 10. The particular bridge circuitillustrated in FIG. 1, is specifically arranged to detect impurities inboth helium and argon atmospheres as well as to detect leakage ofreactive gases into an evacuated enclosure.

The bridge circuit comprises one leg connected between the inputterminals 15 and 16 consisting of the filament 11, fixed resistance 17,and fixed resistances 18, 20 and 21 all connected essentially in seriesbetween the terminals 15 and 16. As shown in FIG. 1, resistances 18, 20and 21 are arranged on a three position selector switch 22 such thatresistance 18 or resistances 18 and 20 or 18, 20 and 21 may be includedin the series circuit simply by setting the movable contact 23 to one orthe other of the three fixed contacts. The purpose of this combinationof resistances will be made clear in the description of the method.

The second leg of the bridge circuit connected between the terminals 15and 16 comprises a variable bias resistor 24, a variable bias trimmerresistor 25 and three fixed resistances 26, 27 and 28 which are arrangedon a three position selector switch 33 such that the resistance 26 orthe resistances 26 and 27 or the resistances 26, 27 and 28 may beinserted in series with the bias and trimmer resistances 24 and 25simply by setting the movable contact 31 to any one of the three fixedcontacts of the switch 30. As will appear in the description of themethod, there are cooperative relations between resistances, 18 and 25,resistances 20 and 27 and resistances 21 and 28. Accordingly, forconvenience of operation, the movable contact 23 of the switch 22 andmovable contact 31 of switch 30 may be ganged together as indicatedschematically by the broken line 32..

The particular apparatus illustrated schematically in FIG. 1 is adaptedto be energized from any convenient 115 volt source 33 through a programtimer 34. One lead 35 from the source 33 is connected through the timerto the input terminal 16 of the bridge. The other lead 36 from thesource 33 is connected to the movable contact arm 41 of a three positionswitch 37, having fixed contacts 38, 39 and 46. The timer is arranged byconventional means to step the movable contact arm from one of the fixedcontacts to the other at predetermined intervals as will subsequently beexplained in greater detail. The fixed contact 38 is connected through aresistance 42 to a timer :output lead 43 and fixed contact 39 isconnected directly to output lead 43. The lead 43 is connected at itsopposite end to input terminal 15 of the bridge. The-third fixed contact40 of the switch 37 is connected through the output lead 44 to thejunction 45 between resistances 17 and 18 in the first leg of the bridgecircuit.

The timer is also provided with a suitable visible or audible signalingdevice, such as bell 46, which is arranged to indicate to the operatorof the apparatus the beginning and end of the significant time periodsinvolved in the practice of the method.

It will be obvious to those skilled in the art that the switchingfunctions to be performed in connection with the various steps of themethod may be made semiautomatic or wholly automatic simply by arrangingthe timer to actuate electromechanical or electronic relay devices atthe prescribed time. Elaborations of this kind are wholly within theskill of the art and, in the interests of simplicity, will not bedescribed here.

The bridge circuit is also provided with bridge balance detecting meanswhich is, in this particular embodiment, a milliammeter. The meter isconnected in series with a resistance 48 and a solenoid actuated switch50 between the junction 45 of resistances 17 and 18 in the first leg ofthe bridge and the junction 51 of resistance 25 and resistance 26 in thesecond leg of the bridge. The solenoid switch may be any suitable typewhich is spring-biased to the open position when tie-energized. Theresistance 48 is shunted by a manually operated switch 52. The actuatingsolenoid 53 of switch 50 is connected between the junction 45 in thefirst leg of the bridge and the input terminal 15 of the bridge.

Titanium, tantalum, tungsten, niobium and alloys of these metals aretypical filament materials. The filament diameter is not critical butshould be made as small as is convenient to the end that the rate atwhich the reactive component affects the filament resistance ismaximized. Furthermore, the length of the filament is preferably madeshort enough so that the resistance of the filament is less than aboutone tenth of the sum of the resistor 17 and the selected seriescombination of resistors 18, 20 and 21'. This will tend to cause thefilament current to remain nearly constant even though the filamentresistance may increase considerably during the test.

The resistance values used in the apparatus illustrated in FIG. 1 areselected so that, when the inert gas being tested is helium, the properballast resistance comprises the series combination of resistances 17and 18.Similarly, when the nominally inert gas being tested is argon,the series combination of resistances 17, 18 and 20 provide-s the properballast. The apparatus is also adapted to test evacuated enclosures forin-leakage of reactive gases. For this purpose the series combination ofresistances 17, 18, 2t) and 21 provide suitable ballast for thefilament.

The resistance values of resistors 26, 27 and 28 in the second leg ofthe bridge are approximately five times the resistances of resistors 18,20 and 21, respectively, in the first leg. This ratio is not critical,but represents a balance between the cost of the resistance and thesensitivity of the null balance detector.

Variable resistors 24 and 25 in the second leg of the bridge arevariable and have maximum values such that the bridge output voltage maybe adjusted to zero by variation of the effective portions of theseresistors.

The null balance indicator 47, is as previously stated, a milliammeterwhich,in this particular circuit, reads one milliampere at full scaledeflection. Resistance 48 in series with the milliammeter has a value'whirh is sufficient to limit the current through the meterto onemilliampere at maximum bridge imbalance. Switch 52 short circuits theresistance 48 to provide maximum bridge sensitivity at conditionsapproaching bridge balance. The solenoid actuated switch 50 is includedto protect the meter in casethe filament breaks before the end of thetest, or was inadvertently not connected before starting the test,either of which conditions would result in a large surge of currentthrough the meter.

Resistance values given in the following table are illustrative only andare suitable for use'with a titanium filament one inch long and 0.0035inch in diameter. I have found that it is advantageous to use resistorshaving rela tively high thermal capacity, i.e. resistors rated at 5watts or more.

Resistor reference No: Resistance (ohms) My new method for detecting thepresence of chemically reactive components, or gaseous impurities, in anominally inert gas is based upon my observation of a novel relationbetween the presence and concentration of a reactive component of agaseous atmosphere and the increase with time of the resistance of afilament which is heated in the presence of the reactive component inorder to accelerate the susceptibility of the filament to chemicalreaction with such reactive component. By

detecting the rate of change of resistance of the filament with respectto time during the initial minutes of the period of heating thefilament, as opposed to time periods. on the order of hours required byprior methods, I obtain an accurate measure of the concentration of thereactive component. My tests have shown that ifv the filament current isheld constant at an appropriate value, the filament resistance increaseswith time in a nonlinear fashion, the rate of increase being greatest atthe start. The general shape of the curve representing this nonlinearrelation between resistance and the time during whichthe filament isheated is independent of the concentration of the reactive component inthe nominally inert gas, but the time scale is contracted or expandeddepending on Whether the concentration of the reactive component islarge or small. As a result, the increase in filament resistanceoccurring in a predetermined period as short as one minute can be usedas a measure of the concentration of the reactive component of thenominally inert gas. FIG. 2 shows a plot of the increase in filamentresistance (in ohms) during the first minute of exposure at constantcurrent, as a function of the concentration of the reactive component.

I will now describe a particular application of my new method as it hasbeen used to determine the oxygen content of a nominally inert gas. Theseveral steps to be performed in this particular application of themethod are facilitated by use of the apparatus described above andillustrated in FIG. 1.

The following description assumes that the enclosure contains anominally inert gas such as helium having some unknown concentration ofa reactive component, such as oxygen, which for many purposes would beconsidered as an impurity in helium. The discussion also assumes that asuitable titanium filament 11 is in place in the enclosure 10 and isconnected in the bridge circuit as is shown in FIG. 1.

The test is started by opening shunting switch 52 and then actuating theprogram timer 34 to connect the source 33 for a short period of time,for example 5 seconds, to the bridge input terminals 15 and 16 throughthe resistance 42 in the timer. The presence of resistance 42 in thecircuit results in an input voltage being applied to the bridge and thefilament. This reduced voltage is selected so that the temperature ofthe filament is raised to about 600 F. to expel any gaseous impuritieswhich may have been adsorbed on the filament surface. At the end of theinitial preheat period the timer sounds the bell 46 and switches thecontact arm 41 from the contact 38 to the contact 39 where full sourcevoltage is connected to the input terminals of the bridge. With trimmerresistor 25 set at zero, the operator now adjusts bias resistor 24 untilthe null balance indicator reads zero, first with switch 52 open andthen with it closed. At the end of about 15 seconds (20 seconds from thestart of the test) which is signaled by the bell 46, the operator leavesvariable resistor 24- as is and then balances any rise in filamentresistance by adjusting trimmer resistor 25 to hold the meter at zero.Resistor 25 may conveniently be precision calibrated to have a directdigital read-out of its resistance to any setting.

At the end of 60 seconds (80 seconds from the beginning of the test),again signaled by the sound of the bell, the operator leaves variableresistor 25 at its last setting and opens switch 52. In five moreseconds the timer switches the movable contact arm 41 of switch 37 tothe fixed contact 40. This de-energizes the solenoid operated switch 50to disconnect the milliammeter 47 and shunts the source voltage around aportion of the first leg of the bridge to the junction 45 which causes asudden increase in the filament current to a value which causes thefilament to melt. The timer then de-energizes the bridge and resets tothe beginning of the program.

The final value indicated by resistor 25 is directly proportional to theincrease in resistance of the filament during the one minute test periodand can be converted to total oxygen content of the inert gas byreference to a predetermined calibration curve.

The method may be repeated at will merely by replacing the filament 11.

In the event that the inert gas being tested is argon, the gang switches22 and would be set so that the movable contact arm 23 of switch 22 isconnected to the fixed contact at the junction of resistances 20 and 21,and the movable contact arm 31 of switch 30 is connected to the fixedcontact at the junction of resistances 27 and 28. If the inertatmosphere is nominally a vacuum, the movabel contact arms would be setto the fixed contacts at the remote ends of resistances 21 and 28,respectively.

The null balancing operation performed by the operator in accordancewith the foregoing description, could be accomplished automatically withcommercially available instruments, and the rise in resistance with timecould be recorded as a permanent graphic record if desired. If tungstenis used as the filament material, the instrument will also measure thetotal oxygen content of nitrogen, the latter being used as an inert gasfor some purposes. These and other variations in the details of themethod and apparatus described in the foregoing specification will occurto those skilled in the art. Accordingly, the scope of the invention isnot intended to be limited by these details, rather the invention isdefined in the following claims.

In the claims:

1. The method of detecting the presence of and determining theconcentration of a gaseous impurity in a nominally inert gaseousatmosphere, comprising the steps of:

(at) introducing an electrically resistive metallic filament into thenominally inert gaseous atmosphere, said filament being chemicallyreactive with such gaseous impurity;

(b) heating said filament to accelerate the susceptibility thereof tochemical reaction with such gaseous impurity; and

(c) detecting the rate of change of resistance with respect to time ofsaid filament during the initial minutes of the period of heating thefilament, wherein said rate of change is related to and indicative ofthe concentration of such gaseous impurity.

2. The method according to claim 1 and in which said filament is heatedduring the step of detecting the change of resistance by means of asubstantially constant electric current through said filament.

3. The method of detecting the presence of and determining theconcentration of a gaseous impurity in a nominally inert gaseousatmosphere, comprising the steps of:

(a) introducing an electrically resistive metallic filament into thenominally inert gaseous atmosphere, said filament being chemicallyreactive with such gaseous impurity;

(b) preheating said filament to a temperature suflicient to vaporizeimpurities in the surface of said filament and insufiicient toappreciably increase the susceptibility of the filament to chemicalreaction with such gaseous impurity;

(c) heating said filament by means of a substantially constant electriccurrent therethrough to a temperature sufficient to accelerate thesusceptibility thereof to chemical reaction with such gaseous impurity;and

(d) detecting the rate of change of resistance with respect to time ofsaid filament following said preheating and during the initial minutesof the period of heating the filament, wherein said rate of change isrelated to and indicative of the concentration of such gaseous impurity.

References Cited UNITED STATES PATENTS 2,219,540 10/ 1940 Miller 23-2322,251,751 8/1941 Minter 73-27 2,298,288 10/1942 Gerrish et al. 23-2322,349,250 5/1944 Doan 23-232 2,888,330 5/1959 Kaplf 23-232 2,904,4069/1959 Moore 23-232 3,222,920 12/1965 Marsh et al 324-71 OTHERREFERENCES Stormont, D. H., The Oil and Gas Journal 55 (3) January 21,1957, pages -87 relied on.

MORRIS O. WOLK, Primary Examiner. R. M. REESE, Assistant Examiner.

1. THE METHOD OF DETECTING THE PRESENCE OF AN DETERMINING THECONCENTRATION OF A GASEOUS IMPURITY IN A NOMINALLY INERT GASEOUSATMOSPHERE, COMPRISING THE STEPS OF: (A) INTRODUCING AN ELECTRICALLYRESISTIVE METALLIC FILAMENT INTO THE NOMINALLY INERT GASEOUS ATMOSPHERESAID FILAMENT BEING CHEMICALLY REACTIVE WITH SUCH GASEOUS IMPURITY; (B)HEATING SAID FILAMENT TO ACCELERATE THE SUSCEPTIBILITY THEREOF TOCHEMICAL REACTION WITH SUCH GASEOUS IMPURITY; AND (C) DETECTING THE RATEOF CHANGE OF RESISTANCE WITH RESPECT TO TIME OF SAID FILAMENT DURING THEINITIAL MINUTES OF THE PERIOD OF HEATING FILAMENT, WHEREIN SAID RATE OFCHANGE IS RELATED TO AND INDICATIVE OF THE CONCENTRATION OF SUCH GASEOUSIMPURITY.